Particle-area dependence of mineral dust in the immersion mode: investigations with freely suspended drops in an acoustic levitator

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Particle-area dependence of mineral dust in the immersion mode: investigations with freely suspended drops in an acoustic levitator

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The lack of most of these methods is that the drops are not freely suspended in air at their terminal velocities during freezing except for vertical wind tunnels; contactfree levitation at least is achieved by electrodynamical and acoustic drop levitators.In contrast to electrodynamical drop levitators, the presence of electrical charges on the drops which might affect the freezing process are not required in acoustic drop levitators.The advantage over measurements in a vertical wind tunnel is that the drops can be levitated for long time periods and are very still which allows to directly determine the surface temperature of the drops during the entire freezing process.The majority of the experiments were performed with two types of illite particles, some studies were undertaken with montmorillonite.For comparisons, measurements with one particle type were additionally performed in the Mainz vertical wind tunnel.Introduction

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Acoustic levitator
The acoustic levitator was also used in two earlier studies of the freezing of supercooled binary and ternary solution drops (Ettner et al., 2004;Diehl et al., 2009).As the freezing of solution drops requires very low ambient temperatures, these experiments were performed by moving the acoustic trap down into a deep freezer with temperatures reaching as low as −85 • C.During the present experiments, the acoustic levitator was installed inside a walk-in cold chamber which achieves ambient temperatures down to −35 • C.These are sufficient for heterogeneous freezing of water drops.As an improvement to the earlier experiments, an infra-red thermometer was used to determine the drop surface temperature.

Instruments and data acquisition
The employed acoustic levitator is the type APOS BA 10 from the company TEC5 as shown in Fig. 1.Inside the trap an ultrasonic wave is produced by a piezoelectric oscillator and reflected by a concave Teflon reflector plate.The interference generates a standing wave with five nodes between the radiator and the reflector.While the source is fixed the reflector is movable mounted on a micrometer screw.In this way the distance between the source and the reflector can be varied by several millimetres to achieve optimal reflector separation at the operating frequency and temperature.Also, optimally, at the third of the five existing nodes drops with diameters between 100 µm and 3 mm can be levitated.The trap was surrounded by an acrylic glass cylinder to protect the levitated drops from the outside air motions of the walk-in cold chamber in order to establish stable temperature conditions during the experiments.The setup in the cold chamber (see Fig. 2) includes the acoustic levitator, a platinum-resistor thermometer Pt100, a digital video camera (FireWire-CAM-011H from Phytec), and an infra-red thermometer (KT 19.82 II from Heitronics).The Pt100 sensor was located Introduction

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Full in the vicinity of the drop to measure the ambient temperature.The video camera and the infra-red thermometer were mounted on adjustable laboratory stages and arranged around the levitator on sliding rails so that their height and distance to the suspended drop could be adjusted.The video camera allowed a visual observation of the freezing process and size determination of the freezing drops.The infra-red thermometer was used to measure the surface temperature of the freezing drops with an accuracy of 0.7 K.It was calibrated before each series of experiments by use of a Pt100 as reference.For the calibration the Pt100 was coated with a layer of water by dipping it in water which subsequently froze bellow 0 • C. The ice covered temperature sensor represents the drop surface emissivity in a more realistic way.
The advantage of the acoustic levitator is that the drops are held stationary with a positional precision which can not be achieved in wind tunnel experiments.When the ultra-sonic field is kept well isolated from ambient air disturbances, no drop movements are visible with the naked eye.This allows performing direct and contact-free temperature measurements of the drop surface during the entire freezing process.As the area of surface temperature observation is a circular spot of approximately 1 mm in diameter a spherical drop shape is desired but the ultra-sonic field causes an oblate deformation of the drop.This deformation is minimized by lowering the input power to the radiator or by increasing the distance between the radiator and the reflector plate which indirectly reduces the power of the ultra-sonic standing wave field.On the other hand, both adjustments lead to a reduced stability of the floated drop.Thus, these two factors, area of temperature observation and stability of the floated drop, require a compromise between a more stable flattened drop and a less stable spherical drop.Another disturbance is the tendency of the drops to oscillate, in particular directly after inserting the drop into the levitator.By temporarily increasing the distance between the radiator and the reflector plate, the initiated oscillations are restrained.
All measuring instruments were arranged around the acoustic levitator inside the cold chamber.Several windows in the protecting acrylic glass tube allow the observation of the levitated drops: a zinc selenide lens for the infra-red thermometer, an optical filter Introduction

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Full for the video camera, a hole for the Pt100 sensor, and, furthermore, a shutter to inject the drops.A Teflon-coated needle from which the drops could be easily released was used to place the drops at the ultrasonic standing wave node.
To control, record, and store the experimental parameters, a PC with the application software LabView was used.The signals of the video camera were monitored on the outside PC so that the drop could be observed on-line during freezing.The transition from the liquid to the ice phase became visible from the change of the transparent liquid drop to an opaque frozen drop as recorded from the video camera but it was clearly defined by the temporal development of the drop surface temperature recorded by the infra-red thermometer.

Experimental procedure
The investigated drops had sizes of 1 mm radius which is larger than cloud drops but is in the range of raindrops (Pruppacher and Klett, 1997).However, as mentioned above, this drop size was required to perform the measurement of the drop surface temperature.Prior to the heterogeneous freezing experiments, the levitator was cleaned by rinsing with purified water and blank checks were performed.Drops generated from pure distilled and de-ionised water of 1 mm radius did not freeze within the experimental time periods of less than a minute at temperatures above −35 • C which is to be expected when no ice nuclei are present (Pruppacher and Klett, 1997).
The measurement of the drop surface temperature allows to clearly determining the onset of freezing.Figure 3 shows an example for the temperature development during freezing.According to Hindmarsh et al. (2003) it can be divided into four stages: 1. supercooling stage before the phase change sets in; here the temperature decreases.2. Recalescence stage which is characterised by a sudden temperature increase.Latent heat is released when the phase change is initiated so that the surface temperature of the drop reaches almost 0 • C. 3. Freezing stage which is a rather long time period where the entire drop freezes; at the end, the temperature decreases again; 4. Cooling stage after the drop is entirely frozen.Introduction

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Full For the freezing experiments the cold chamber was always pre-cooled to a low temperature around −30 ± 1 • C. Thus, the cooling rate of the drops, i.e. the rate by which the drop temperature was adapted to the ambient temperature was the same during all experiments.The development of the drop surface temperature with time was measured several times with pure water drops.The drops reached a lowest temperature of −27 ± 0.7 • C only because the levitator was isolated against the cold chamber air (see Sect. 2.1.1).From the measurements the following equation was derived to describe the drop temperature T drop (t) in • C: with the time t in s and the result is shown in Fig. 4. Drops were generated from distilled water which was mixed with the selected mineral dust particles in defined amounts.
During the experiments, this solution was continuously stirred to avoid a settling and agglomeration of the particles.It was kept inside the cold chamber at a constant temperature slightly above 0 • C. Individual drops were levitated one after another until they froze and the freezing temperatures, i.e. the lowest surface temperatures were recorded.For each particle type and concentration, approximately 100 drops were observed.

Vertical wind tunnel
In the Mainz vertical wind tunnel drops from micrometer to millimetre sizes are freely floated at their terminal velocities in a vertical air stream.Thus, ventilation and heat transfer are similar to the situation as in the real atmosphere.The experiments were performed similar as described in Diehl et al. (2002) andv. Blohn et al. (2005) where immersion freezing of pollen was investigated.Detailed descriptions of the wind tunnel are given therein and in the review paper of Diehl et al. (2011).To perform ice nucleating experiments, the tunnel can be cooled down to −30 • C. Air sucked in from outside is passed through particle filters to avoid the presence of possible ice nuclei during the 12893 Introduction

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Full experiments.This was ensured in the way that it was proven that drops containing no particles did not freeze within the investigated temperature range.For the immersion freezing experiments, the wind tunnel was pre-cooled to certain temperatures in steps of 1 K so that, in contrast to the acoustic trap experiments, drop freezing was observed at constant temperatures.Particles were mixed with distilled water in defined concentrations and drops formed from this mixture were injected into the wind tunnel.For each temperature and particle type, 40 to 50 drops were investigated.The onset of freezing was determined by observation and is characterized by an opaque look of the drops and a different floating behaviour.
As an improvement to earlier measurements, the fractions of frozen drops were determined time-resolved.When a drop started floating, a time recording was started until the drop froze.The total observation time per drop was 30 s, i.e. drops which did not freeze within this time period were counted as unfrozen.Wind speed, temperature, and relative humidity of the tunnel air were recorded continuously.These parameters were required to calculate afterwards the drop sizes and the drop surface temperatures.
As the air velocity in the tunnel must be equal to the terminal velocity of the floated drop to keep it floating in the observation section the drop size could be determined from the recorded wind speed (v. Blohn et al., 2005).In order to obtain similar sized drops for the experiments only those drops were observed which were suspended in a rather narrow wind speed range of 2.70 ± 0.25 m s −1 .From this terminal velocity the average drop size was calculated according to Pruppacher and Klett (1997) to 340 ± 30 µm.The drops observed in the wind tunnel were smaller than the ones used in the acoustic trap but still larger than cloud droplets.This was a required compromise as drop freezing was determined by visual observation.According to Pruppacher and Klett (1997, Chapt. 13) the actual drop temperature while suspended in the wind tunnel and the adaptation time, i.e. the time until the drop temperature is equal to the ambient temperature were calculated from the ambient tunnel air temperature and the dew point of the tunnel air.As shown in Fig. 5, it takes between 3 and 5 s until the temperature of a 370 µm drop reaches the ambient temperature.For the present experimental Introduction

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Full conditions, an adaptation time of 4 s was assumed.The measurement accuracy of the drop temperature was estimated as ±1 K.

Particle samples
Several particle samples were used in the experiments.As first test ice nuclei in the acoustic levitator montmorillonite K10 particles were selected which are commercially available and characterised by a specific surface area of 220 m 2 g −1 (Sigma Aldrich).The majority of the experiments were performed with illite particles.Two types of illite were used: illite IMt1 and illite NX.Illite IMt1 is available from the Clay Mineral Society and according to the manufacturer is consists of 85 to 90 % illite and 10 to 15 % quartz.
Particle diameters range from less than 0.2 µm to larger than 63 µm with a maximum of the number size distribution at 1.2 µm (Köster, 1996).Its specific surface area was determined as 31.7 m 2 g −1 (Hiranuma and Möhler, 2013).Illite NX was obtained from B + M Nottenkämper as Arginotec ® NX and is composed by 86 % illite, 10 % kaolinite, 4 % calcite, and traces of quartz and feldspar.Its number size spectrum shows a maximum at 0.3 µm diameter, particle sizes are varying from 0.03 to 10 µm and the specific surface area is 124.4 m 2 g −1 (Hiranuma and Möhler, 2013).The latter illite type was se-

Results from acoustic levitator experiments
The particles immersed in the drops were evaluated in terms of particle surface area per drop.These were calculated from the particle concentrations in the stock solution and the average drop size.With illite IMt1 and illite NX particles, three different particle concentrations were investigated, with montmorillonite K10 only one particle concentration.Two illite NX particle concentrations were investigated at the vertical wind tunnel also.The cases are listed in Table 1.
The results indicate that immersion freezing is dependant on the particle surface area in the drop.This confirms the findings of other recent studies as, e.g., Murray et al. (2011)  Figure 8 demonstrates the freezing behaviour of montmorillonite K10 in comparison to illite.Although it was present in the drops with a particle surface area of 2.5 × 10 −4 m 2 drop −1 , i.e. one order of magnitude higher than in the highest cases of illite IMt1 and NX (7.0×10 −5 and 7.1×10 −5 m 2 drop −1 , respectively), its median freezing temperature does not exceed the ones of illite IMt1 and NX (−20.6 • C in comparison to −18.8 • C and −19.7 • C, see Table 1).

Results from wind tunnel experiments
Figure 9 gives the fractions of frozen drops as function of temperature for illite NX as measured in the wind tunnel for comparable particle surface areas in the drops (represented by same colours as before in Fig. 7).The figure shows accumulated data for the total observation time of 30 s.The median freezing temperatures were −20.8±1 • C and −19.2±1 • C for particle surface areas of 5.1×10 −6 m 2 drop −1 and 5.1×10 −5 m 2 drop −1 , respectively.Thus, the deviations between the results from the two techniques are within the measurement accuracies (see Table 1).As the experiments at the wind tunnel were performed at constant temperatures, the time-

Comparison to literature data
The immersion freezing behaviour of illite NX was investigated by Broadley et al. (2012) for particle surface areas between 1 × 10 −11 m 2 to 3 × 10 −8 m 2 corresponding to drops Introduction

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Full with radii between 5 and 40 µm.Thus, in their experiments the drop volumes were at least 4 orders of magnitudes smaller and so were the particle surface areas.In Fig. 12 the median freezing temperatures as determined by Broadley et al. (2012) and the present results (from acoustic levitator and wind tunnel) are shown in green and blue, respectively.Broadley et al. ( 2012) found an approximately linear increase of the median freezing temperature with the log of the particle surface area per drop for particle surface areas smaller than 6 × 10 −10 m 2 (solid green line in Fig. 12).Afterwards, as indicated by the dotted green line, for small drops T 50 stays nearly constant but it further increases for larger drops containing a larger particle surface area.This tendency is obvious for the present cases with significantly larger drops and larger particle surface areas: the median freezing approximately increases with the log of the particle surface area.An extrapolation of the regression line (blue solid line in Fig. 12) towards smaller particle surface areas (blue dotted line) comes into the region of the Broadley et al. ( 2012) results.

Application of heterogeneous freezing models
Two models were applied to describe heterogeneous freezing, the time-dependant stochastic model and the singular model which neglects time dependence in comparison to particle variability.An important advantage of the singular model is that it allows simplifying the description of ice formation in cloud models (Vali, 2008).

Stochastic model
The stochastic model is initiated by the description of homogeneous freezing where it is assumed that small clusters randomly form in the supercooled liquid phase, only some of them growing further to macroscopic crystals (e.g., Pruppacher and Klett, 1997;Murray et al., 2010).In case that a solid surface is present this stabilizes the clusters of the ice phase.To extend the homogeneous stochastic model to heterogeneous ice nucleation by a single species it is assumed that the nucleating probability is similar for Introduction

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Full all drops of a population.Thus, the number of drops ∆n fr freezing heterogeneously in a time period ∆t is given by (Murray et al., 2011): with N total the total number of observed drops, s the particle surface area immersed in the drops, and J(T ) the nucleation rate coefficient per unit particle surface area and time.With f ice meaning the fraction of frozen drops as determined in the experiments, and the freezing time t, the nucleation rate coefficients J(T ) per unit particle surface area and time can be calculated according to Murray et al. (2011): As Eq. ( 4) is valid for constant temperatures it is applicable to the wind tunnel experiments only.Following Koop et al. (1997) the freezing time t can be interpreted as the total cumulative observation time, i.e. 30 s as due to the stochastic model each freezing event is independent on the number of previous trials and the different freezing events are not dependant on each other.Figure 13 shows the nucleation rate coefficients J(T ) for illite NX present with 5.1×10 −6 m 2 per drop and 5.1×10 −5 m 2 per drop derived from the wind tunnel experiments by using Eq. ( 4).For the limited ranges of particle surface area and temperature the nucleation rate coefficients can be described by a single line within the experimental errors (black line in Fig. 13) thus indicating that the log of the nucleation rate coefficient increases with temperature.To look closer to the time dependence of freezing, the liquid ratio, i.e. the fractions of drops which remain liquid were calculated as function of time.From Eq. ( 2) it can be Introduction

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Full ing the cooling stage of the drops were not considered, that means freezing events during the first 4 s (i.e. on the left-hand-side of the vertical line in the figures).One can note that except for one case (at −21 • C with s = 5.1 × 10 −6 m 2 per drop) the data very well follow straight lines indicating the exponential decay of the liquid drops with time at constant temperatures as predicted by Eq. ( 5).This confirms that nucleation events during the wind tunnel experiments were stochastic processes affected by a single nucleation site type.
To apply the stochastic model to the data measured with the acoustic trap one has to consider the effect of the non-linear cooling rate.For that purpose Eq. ( 4) was modified as follows.It was assumed that during temperature changes of 1 K the cooling rate was nearly constant.For each temperature in steps of 1 K the total cooling time t was calculated according to Eq. (1).Afterwards, for each temperature change ∆T the required time ∆t was calculated and the cooling rate γ(T ) was determined from with ∆T = 1 K. Thus, for a temperature change from T 1 to T 2 there is a cooling rate according to Eq. ( 6) and the change of the frozen fraction is given by

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Full considering that N total is constant.Analogue to Eq. ( 2), the number of drops ∆n fr freezing heterogeneously during a temperature change ∆T is given by with the cooling rate γ in units of [K s −1 ] and the nucleation rate coefficient J(T ) in units of [area −1 s −1 ].Thus, J(T ) can be calculated by with ∆T = 1 K.
A comparison of the nucleation rate coefficients derived from acoustic levitator and wind tunnel measurements is given in Fig. 16.Results from wind tunnel experiments (derived according to Eq. 4) are shown with their regression line (solid line in black) as in Fig. 13.The dotted black line gives the extrapolation of the regression line towards higher and lower temperatures.Results from acoustic trap experiments were calculated by using the modified Eq. ( 9).They show a rather large scatter around the regression line; however, in the temperature range below −17 • C they follow the same trend.At higher temperatures the data are located definitely above the extrapolated regression line.This indicates that in the acoustic levitator experiments the freezing rate of the drops during the first 15 s of cooling is overestimated because of the very fast cooling rate in the beginning.

Singular model
Besides the stochastic model, it is suggested that heterogeneous freezing is dominated by the nucleating characteristics of ice-active sites and, thus, only dependant on temperature while the time-dependence, i.e. the stochastic nature of the freezing 12901 Introduction

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Full process becomes negligible (Vali, 2008).The singular model is based on the assumption that critical clusters form on ice-active sites at characteristic temperatures so that freezing takes place as soon as the characteristic temperature is reached without any time dependence.Thus, if the temperature is held constant one would not observe further freezing events.However, this is not consistent with the present wind tunnel observations and previous measurements (e.g., Murray et al., 2011;Broadley et al., 2012).
According to the singular model of heterogeneous freezing the surface density of active sites n s (T ) per unit particle surface area was determined from the present data as follows (Murray et al., 2011): Figure 17 shows the surface densities of active sites n s for illite NX from both techniques, marked by different colours, for all investigated particle surface areas per drop.The two data sets show a very good agreement within the measurement errors and are represented by one third order polynomial regression curve (solid black line).The data measured with illite IMt1 (Fig. 18) follow a somewhat different trend so that they are represented by another third order polynomial regression curve.A comparison of these regression curves as shown in Fig. 19  The two techniques used in the present investigations both allow freely suspending single drops without electrical charges and wall or substrate contacts but having the disadvantage that the drop sizes are larger than typical cloud drops.In the wind tunnel, the drops reach equilibrium with the ambient temperature within 3 to 5 s and, afterwards, can be kept at a constant temperature during observation.Thus, the time dependence of the freezing process is clearly indicated.In the acoustic levitator, it is not possible to cool down the drops within seconds because of the larger drop volume and the missing ventilated heat transfer.Therefore, they cool down more slowly exchanging heat with the ambient air in the cold chamber which, because of the large drop volume, results in a non-linear cooling rate.The freezing time is coupled to the freezing temperature and cannot be used to determine nucleation rate coefficients.
Because of the constant temperatures during wind tunnel experiments, the stochastic model was applied to those results with the use of the total observation time as freezing time.For the measurements in the acoustic trap, a modification was required to account for the non-linear cooling rate of the drops.This was undertaken by using piecewise linear cooling rates.The resulted nucleation rate coefficients agree well with the ones derived from wind tunnel experiments for temperatures lower than −17 • C.
Although the singular model which neglects time dependence of nucleation does not account for the observations that freezing proceeds with time at constant temperatures, its application to the present wind tunnel and acoustic trap results show a good agreement between the two experimental techniques.The derived number densities of active sites can be used in cloud models to easily describe ice formation in the immersion mode.However, parameterizations applicable in cloud models are not given in the present article as this would be presented in a forthcoming paper.The special mineral dust type illite NX is currently being investigated by a number of different experimental Introduction

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Full A compilation of all results from the entire INUIT community with a general parameterization applicable to model simulations will be presented in a future joint publication.
In spite of some deficiencies the use of the acoustic levitator has a number of essential advantages.It is a small transportable instrument and can be easily installed.
It does not require a huge amount of air flow and, thus, energy to establish drop floating and is, therefore, much more economical in its operation.Although in the present experiments the levitator was placed inside a walk-in cold chamber, it might as well be placed inside a table top cold box.The possibility to directly measure the drop temperature with an infra-red thermometer allows to clearly defining the onset of freezing.
In particular for nucleation processes, flow hydrodynamics and ventilated heat transfer as it happens in the wind tunnel are not deciding factors as much as the temperature and the cooling rate.If required, a small design modification could bring additional ventilated heat transfer.Thus, the acoustic levitator presents a good alternative to other methods to investigate drop freezing in the immersion mode.Introduction

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Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | lected as a test aerosol to be used in the German research group INUIT and is currently investigated by several different techniques (see INUIT website: www.ice-nuclei.de).According to Broadley et al. (2012) illite NX might be used as a proxy for atmospheric dust as its composition is rather close to atmospheric samples of mineral dust.
surface area per drop by two orders of magnitude.Differences between the two illite types with comparable particle surface areas are not significant as they are only slightly higher than the measurement accuracies (±0.7 K): the median freezing temperatures of illite IMt1 are −23.6 ± 0.7 • C and −18.8 ± 0.7 • C for particle surface areas of 3.5 × 10 −6 m 2 drop −1 and 7.0 × 10 −5 m 2 drop −1 , respectively, for illite NX median freezing temperatures of −22.8 ± 0.7 • C and −19.7 ± 0.7 • C were measured for particle concentrations of 7.1 × 10 −6 m 2 drop −1 and 7.1 × 10 −5 m 2 drop −1 , respectively.Discussion Paper | Discussion Paper | Discussion Paper | resolved freezing fractions are shown in Figs. 10 and 11 for the two investigated particle surface areas.The results indicate that even after an adaptation time of 4 s (marked as black vertical line in the figures) the frozen fraction still increases.At a given temperature, e.g., −21 • C freezing proceeds more slowly with the lower particle surface area 5.1 × 10 −6 m 2 per drop (see blue symbols in Figs. 10 and 11).So it is obvious that at lower temperatures and with higher particle surface areas per drop freezing proceeds faster.Such behaviour has been observed also by Murray et al. (2011) and Broadley et al. (2012) in isothermal experiments.
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | are given in Figs. 14 and 15 for 5.1×10 −6 m 2 per drop and 5.1×10 −5 m 2 per drop, respectively, for different temperatures.Cases where freezing took place dur- Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (blue and black symbols and lines) indicates that they cross each other at a temperature of −19 • C. The deviations in the other temperature ranges are in most cases slightly outside the measurement errors.In contrast, the results from montmorillonite K10, additionally shown in Fig. 19 in red, lie below the curves for illite and follow another trend.Although data are available only for a limited temperature range this trend indicates a reduced surface density of active sites of montmorillonite which is to be expected according to the results of Atkinson et al. (2013).In general, Figs. 17 to 19 indicate that the lower the temperature the more active sites are available on the ice nucleating material.The nucleation rate coefficients and surface densities of active sites are compared to the results of Broadley et al. (2012) for immersion freezing of illite NX.In Fig. 20, the area of the Broadley et al. (2012) nucleation rate coefficients for various cooling rates and different particle surface areas between 1 × 10 area.The present results are shown as blue and red symbols (results from acoustic levitator and wind tunnel, respectively).Because of higher particle surface areas they are located in a completely different temperature range but the trend towards lower temperatures shows that they approximate the Broadley et al. (2012) results.In Fig. 21 the surface densities of active sites for illite NX from the present experiments (shown as blue and red symbols, polynomial regression curve from the data as solid line) are compared to the results from Broadley et al. (2012) (green solid line as fit to the data, from Broadley et al., 2012).Although there is a gap between the two lines at a temperature of −25 • C their extrapolations would meet each other.These comparisons indicate that the present results very well complement the results from Broadley et al. (2012) as the trends observed for lower particle surface areas are confirmed by the present results in the higher temperature range.4 Summary and conclusions Immersion freezing was found to occur in a temperature range between −14 and −26 • C in dependence of particle type and mass concentration per drop.The rather high freezing temperatures are due to the drop sizes of 340 µm and 1 mm in radius and, thus, large mass concentrations and particle surface areas per drop.The data were evaluated in terms of nucleation rate coefficients and surface densities of active sites according to both the stochastic and singular models of heterogeneous freezing.The present results very well complement earlier results from Broadley et al. (2012) for illite NX as they investigated smaller drops containing less particle surface areas and Discussion Paper | Discussion Paper | Discussion Paper | freezing in a lower temperature range.The other type of illite IMt1 used in the present experiments led to similar results although its composition is somewhat different.The findings for montmorillonite K10 indicate a trend towards a lower surface density of ice-active sites as suggested by Atkinson et al. (2013).
Discussion Paper | Discussion Paper | Discussion Paper | techniques within the frame work of INUIT.

Figure 2 :
Figure 2: Scheme of the experimental setup with the acoustic trap in the cold chamber.

Figure 2 .
Figure 2. Scheme of the experimental setup with the acoustic trap in the cold chamber.

Figure 3 .
Figure 3. Example of the development of the drop temperature with time during freezing.

Figure 4 .
Figure 4. Development of drop temperature during freely floating in the acoustic levitator at a cold chamber ambient temperature of −30 • C.

Figure 5 .
Figure 5. Calculated adaptation times of 370 radius drops while freely floating in the wind tunnel at various ambient temperatures.

Figure 6 :
Figure 6: Immersion freezing of illite IMt1 in the acoustic levitator: frozen fraction of drops as function of temperature for three different particle surface areas per drop.

Figure 6 .
Figure 6.Immersion freezing of illite IMt1 in the acoustic levitator: frozen fraction of drops as function of temperature for three different particle surface areas per drop.

Figure 8 .
Figure 8. Immersion freezing for montmorillonite K10 in comparison to illite: frozen fraction of drops as function of temperature.

Figure 9 .
Figure 9. Immersion freezing of illite NX in the wind tunnel: frozen fraction of drops as function of temperature for two different particle surface areas per drop.Accumulated values within total observation time of 30 s.

Figure 10 .
Figure 10.Time resolved frozen fractions measured in the wind tunnel with illite NX present with 5.1 × 10 −6 m 2 per drop at different temperatures.

Figure 11 .
Figure 11.Time resolved frozen fractions measured in the wind tunnel with illite NX present with 5.1 × 10 −5 m 2 per drop at different temperatures.

Figure 13 :
Figure 13: Nucleation rate coefficients as function of temperature for illite NX for two particle surface areas per drop, investigated in the wind tunnel.

Figure 13 .
Figure 13.Nucleation rate coefficients as function of temperature for illite NX for two particle surface areas per drop, investigated in the wind tunnel.

Figure 15 .
Figure 15.Liquid ratio as function of time from wind tunnel experiments at different temperatures, illite NX present with 5.1 × 10 −5 m 2 per drop.Vertical line: limit of adaptation time of the drops.

Figure 16 :
Figure 16: Nucleation rate coefficients as function of temperature for illite NX for various particle surface areas per drop, determined by two experimental techniques.

Figure 16 .
Figure 16.Nucleation rate coefficients as function of temperature for illite NX for various particle surface areas per drop, determined by two experimental techniques.

Figure 18 :
Figure 18: Surface density of illite IMt1 as function of temperature for various particle surface areas per drop, determined by the acoustic levitator.

Figure 18 .Figure 18 :
Figure 18.Surface density of illite IMt1 as function of temperature for various particle surface areas per drop, determined by the acoustic levitator.

Figure 19 :
Figure 19: Comparison of the surface densities of illite NX and IMt1.

Figure 19 .
Figure 19.Comparison of the surface densities of illite NX and IMt1.

Figure 20 :
Figure 20: Nucleation rate coefficients for illite NX derived from present (shown in blue and red) and previous experiments (indicated as grey region, from Broadley et al., 2012).

Figure 20 .
Figure 20.Nucleation rate coefficients for illite NX derived from present (shown in blue and red) and previous experiments (indicated as grey region, from Broadley et al., 2012).

Table 1 .
Experimental conditions during freezing experiments.